Chemical Attack Evidence Lasts Years, Experts Say

Scientists have discovered that sarin, a deadly nerve agent, can be detected long after its use on the battlefield. In one case, forensic experts went to a Kurdish village in northern Iraq four years after Iraqi warplanes had dropped clusters of bombs there. The experts found a unique chemical signature of the lethal toxin in contaminated soil from bomb craters.

Such findings suggest that the Syrian government would have a hard time hiding evidence if it did indeed use chemical weapons against civilians in a large-scale attack last week. Syrian rebels, Obama administration officials and chemical arms experts have accused President Bashar al-Assad’s forces of using highly toxic chemicals; the government has denied the charges.

Weapon experts say the science of detecting deadly chemicals that can evaporate quickly after strikes has improved over the decades as the United Nations has conducted a dozen investigations of purported attacks. Starting in 1981, its inspectors have studied battlefield evidence in such places as Iraq, Iran, Mozambique, Azerbaijan and now Syria.

“They can pinpoint chemicals long after the fact,” said Amy E. Smithson, an expert on biological and chemical weapons at the Monterey Institute of International Studies. “In past investigations, the inspectors have collected incredibly useful and at times incriminating evidence.”

Identifying the exact makeup of toxic chemicals, especially in the field under less-than-ideal conditions, can be a tricky process rife with false alarms. But sending the field samples to distant laboratories for more thorough analysis can typically provide unambiguous answers to warfare allegations.

Experts say two large, complex instruments lie at the heart of advanced chemical detection: the gas chromatograph and the mass spectrometer. The chromatograph breaks up chemicals into their components, and the spectrometer identifies them by comparing them to libraries of known substances.

With readings from both instruments, scientists in highly specialized laboratories can usually detect the presence of a chemical agent or its breakdown products in a field sample.

“They’re very good,” said David H. Moore, a toxicologist at Battelle Memorial Institute, a nonprofit research group in Columbus, Ohio, and a former official at the United States Army Medical Research Institute of Chemical Defense, at the Aberdeen Proving Ground in Maryland. “If adequate samples are collected, there’s a high probability that they will find conclusive evidence of exposure to chemical warfare agents.”

A global treaty known as the Chemical Weapons Convention of 1993 bars the development, production, stockpiling and use of chemical arms. The Organization for the Prohibition of Chemical Weapons, based in The Hague, polices the treaty and relies on a global network of more than a dozen top laboratories to analyze field samples.

Dr. Moore said he had visited many of the laboratories, including one at the Aberdeen Proving Ground, and judged their work to be excellent.

With Syria, he added, the best evidence for chemical forensics would be blood and tissue samples from victims and survivors that display acute symptoms. Careful analysis of such samples, he said, can reveal “telltale markers.”

Dr. Moore said his own judgment of the Syrian situation, based on viewing pictures of the victims, was that the crippling and killing “clearly looks like the work of a nerve agent.”

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A man and baby in Iraq in 1988. Investigators found evidence of a nerve agent in soil in 1992.CreditIrna, via Agence France-Presse

But he also said the United Nations inspectors could be confounded if, a week after the attack, the “worried well” presented vague symptoms but no solid evidence of chemical exposure.

“The further you are from an incident,” Dr. Moore noted, “the more difficult the investigation will become.”

Another hurdle, experts note, is that Syria has been shelling the area of the massacre in what Secretary of State John Kerry on Monday described as an attempt at “systematically destroying evidence.”

The clock may be ticking not only on environmental clues in Syria, but biological ones as well. Ron G. Manley, a former British military specialist and director of verification for the Organization for the Prohibition of Chemical Weapons, said studies had shown that the human body eventually metabolizes traces of nerve agents, erasing the chemical evidence.

Even so, Dr. Manley said in an interview, British scientists had managed to find unambiguous signs “in blood and urine samples for up to two weeks” after a chemical attack.

The United Nations helped pioneer the investigation of chemical attacks after there were bold claims of mass slaughter with unconventional arms and many looked to the world body for adjudication.

In 1988, for instance, Iran charged that Iraq had used chemical weapons against a number of Iranian villages. In response, the United Nations sent an investigative team that reported clear evidence of chemical warfare.

In 1992, a forensic team assembled by Physicians for Human Rights, based in Boston, and Middle East Watch, a human rights group based in New York, conducted an unusual experiment to see if clear evidence could be uncovered long after a chemical attack. Its scientists went to the Kurdish village in northern Iraq that had been bombed by Iraqi warplanes four years earlier, and they sent field samples to the Chemical and Biological Defense Establishment of the British Ministry of Defense. It found trace evidence of sarin as well as mustard gas, another chemical agent.

Graham Pearson, the establishment’s director general at the time, said the detection showed that “samples collected from appropriate locations can provide evidence of the presence of chemical warfare agents over four years after the attack.”

Before the Syrian investigation, the United Nations inspectors had most recently looked into charges of chemical weapons use in a conflict between Armenia and Azerbaijan. The team found no such evidence.

Weapons experts note that investigations into charges of chemical warfare can at times prove difficult because nerve agents can resemble other chemicals, notably insecticides. Tabun and sarin, both potent nerve agents, were developed in Germany in 1936 and 1938 as insecticides and only later stockpiled for war.

In an interview, Raymond A. Zilinskas, a senior scientist at the Monterey Institute of International Studies and a former United Nations weapons inspector, said one question in Syria was whether the toxic clouds at the site of the massacre came from an industrial accident instead of a chemical weapons attack.

But Dr. Zilinskas said the inspection teams, after decades of experience and access to the latest equipment, most likely had the means to tell the difference between a chemical attack and an accident.

“There’s a fairly good probability that they’ll be able to differentiate between the two,” he said. “A well-equipped unit of inspectors can do a lot.”

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